My invention relates to a method of firing ceramic bodies containing a diffusible metallic oxide such for example as antimony trioxide, Sb.sub.2 O.sub.3. The method of my invention is of particular utility in fabricating electrically resistive ceramic bodies containing a major proportion of zinc oxide, Zn0, and a minor proportion of Sb.sub.2 O.sub.3, among other metallic oxides, and well suited for use as varistors of improved antisurge capability, although I do not wish my invention to be limited to this particular application.
The varistor is a two electrode semiconductor device, sometimes referred to as a voltage depending resistor because of its voltage dependent nonlinear resistance. An essential property of any ceramic material for use as varistors is, therefore, a nonlinear volt ampere characteristic. A ceramic composition containing a major proportion of zinc oxide and minor proportions of other metallic oxides represents one of the most familiar varistor materials meeting this requirement, being capable of withstanding very high voltage surges.
According to the usual conventional practice for the fabrication of varistors, required proportions of the ingredients in finely divided form were first intimately intermingled. Then, after being admixed with an organic binder, tim mixture of the ingredients was molded into disks. A number of such disklike moldings were then fired simultaneously by making them stand edgewise in a row on a horizontally elongate stand defining a furrow of V shaped cross section. Typically, the molding stand was of either aluminum oxide (alumina), AI.sub.2 O.sub.3, or magnesium oxide (magnesia), Mg0.
I have found that this conventional practice is strongly objectionable in cases where the moldings contain a diffusible substance, particularly a low melting point metallic oxide such as Sb.sub.2 O.sub.3. Such a metallic oxide has proved to diffuse readily into the molding stand when the moldings are fired at a temperature above its melting point. Consequently, no matter how intimately the ingredients were intermingled, the diffusible metallic oxide became markedly absent from the neighborhoods of those edge portions of the completed ceramic bodies which had been in contact with the stand.
Such absence of some ingredient from parts of the ceramic bodies can make them totally defective, not usable for their intended applications. Take for example the noted Zn0 ceramic composition containing Sb.sub.2 O.sub.3 as a diffusible metallic oxide. Sb.sub.2 O.sub.3 in this composition is intended to serve the purpose of preventing the crystal grains of zinc oxide from growing too large in size upon firing of the moldings. Actually, I have confirmed that the Zn0 crystal grains grew inordinately large at those portions of the completed ceramic bodies where the content of Sb.sub.2 O.sub.3 was much less than at the other portions. The antisurge capabilities of the varistors having regions of such large crystal grains were much less than those of varistors of the same composition having no such localized regions of reduced Sb.sub.2 O.sub.3 concentration.